//+/-可以将类型将后面跟着的类型转换成数字类型,+-代表正负号
console.log(+'1')//1
console.log(+'-1')//-1
console.log(+true)//1
console.log(+undefined)//NaN
console.log(-'5') //-5
console.log(+null) //0
console.log(-null) //0